The Blackmar-Diemer Gambit is a chess opening that begins with 1. d4 d5 2. e4.

An aggressive chess opening first explored by Armand Edward Blackmar, a relatively little known player in the late 19th century, but mostly advocated by the German master Emil Joseph Diemer (1908-1990). It was therefore named after these two players.

The Blackmar-Diemer Gambit is a true gambit in the sense that Black can hold on to the extra pawn if he so wishes. The accepted gambit usually continues 2...dxe4 3. Nc3 (the immediate 3. f3?! is not good as 3...e5! lets Black catch up in development) and now Black usually plays either 3...Nf6 or 3...Bf5. White will then continue with 4. f3, going permanently down a pawn but with good chances thanks to his superior development after 4...exf3 5. Nxf3 (5. Qxf3!? is also seen, see Halosar Trap).

Note that Black can also decline the gambit with 2...e6 (leading to a French Defense) or 2...c6 (leading to a Caro-Kann Defense). These are however less popular transpositions.

As with most gambits, White aims to achieve rapid development and active posting of his pieces in order to build up a rapid king attack. It is believed that Black is able to defend his position in all variations and consolidate his extra pawn to good chances in the endgame, so this opening is almost never seen in top-level play. It is however somewhat common in amateur play and a trusty weapon of many a club player.